1. Field of the Invention
This invention relates to a sensor array driving circuit for driving an array of sensing elements, and, in particular to a sensor array driving circuit for driving a plurality of photoelectric elements, such as photodiodes, arranged in the form of an array in sequence to obtain a serial output signal converted from light image information.
2. Description of the Prior Art
A sensor array including an N.times.L plurality of photoelectric sensing elements which are grouped into an L plurality of blocks each having the same number, i.e., N, of the photoelectric sensing elements, an L plurality of common electrodes each connected to one end of each of the sensing elements of the corresponding block and an NxL plurality of individual electrodes which are individually connected to the other end of each of the sensing elements is known in the art ( N and L are positive integers). For example, in an image sensing device including a plurality of photoelectric sensing elements arranged in the form of a single array for use in a facsimile machine, the sensing elements arranged in the form of a single array are sequentially activated one at a time from one end to the other to carry out scanning along a single horizontal scanning line sector across the width of an original image which is being transported in a predetermined direction with respect to the sensor array. In this manner, the optically scanned light image information is converted into a serial, electrical image signal.
In such an image sensing device which includes commonly connected sensing elements in each block and which activates the sensing elements one at a time in a predetermined sequence, some measure must be taken to prevent the interactions between the sensing elements when activated from taking place. For example, in accordance with one prior art approach, provision is made of a plurality of switches, mechanical or electronical, which are selectively operated to carry out the required sequential activation, and an array of diodes are provided as connected to the sensing elements to prevent the interactions between the commonly connected sensing elements. Described more in detail, as shown in FIG. 1, this type of prior art image sensing device includes N.times.L number of photoelectric or sensing elements, such as photodiodes, S.sub.11, S.sub.12, . . . , S.sub.LN arranged in the form of one dimensional array 10, which is connected between a d.c. power supply E and ground through L number of make contacts BS.sub.1 -BS.sub.L, N number of make contacts DS.sub.1 -DS.sub.N and L.times.N number of diodes D.sub.11 -D.sub.LN. As shown, each of the individual electrodes connected to the cathodes of the diodes is connected to the corresponding individual electrodes of the other blocks, and the thus connected individual electrodes are connected to the output terminal V.sub.out through respective diodes DO.sub.1 -DO.sub.N.
In the driving circuit of FIG. 1, one of the make, or normally open, contacts BS.sub.1 -BS.sub.L and one of the contacts DS.sub.1 -DS.sub.N are selectively closed to have one of the L.times.N sensing elements S.sub.1 -S.sub.LN activated at a time. The diodes D.sub.11 -D.sub.LN and DO.sub.1 -DO.sub.N function to block unwanted signals from entering into the selected sensing element. In such a prior art structure, it is obvious that (L+1)N number of such blocking diodes are required. However, providing such a large number of diodes in the sensor array 10 is rather complicated and difficult, which tends to push up the manufacturing cost.
FIG. 2 shows another prior art sensor array driving circuit including L.times.N number of sensing elements S.sub.11, S.sub.12, . . . , S.sub.LN, arranged in the form of a single array 10. The sensing elements are divided into L number of blocks and the sensing elements in the same block are commonly connected at one end. These commonly connected ends are connected to L number of transfer switches BS.sub.1 -BS.sub.L, each having its make contact connected to ground and its break contact connected to the d.c. power supply E.
On the other hand, the corresponding sensing elements, one in each of the blocks, are connected to each other at the other ends, which, in turn, are connected to the output terminal V.sub.out through respective operational amplifiers A.sub.1 -A.sub.N and make contacts DS.sub.1 -DS.sub.N. This structure calls for the provision of N number of operational amplifiers A.sub.1 -A.sub.N, each of which has its output connected to its inverting input through a feed back resistor R.sub.f.
In operation, similarly with the case of the circuit shown in FIG. 1, the switches BS.sub.1 -BS.sub.L and the contacts DS.sub.1 -DS.sub.N are selectively operated to have one of the sensing elements activated so that an output signal, whose magnitude is inversely proportional to the electrical resistance of the selected sensing element, may be obtained at the output terminal V.sub.out through the corresponding operational amplifier. With such a structure, the sensing elements may be operated independently from each other without interactions with the other elements. In this second prior art approach, however, a relatively large number of operational amplifiers, which are rather complicated in structure, and the same number of feed back resistors must be provided, so that the overall circuit structure tends to be complex, large in size and expensive to make.